This invention relates to an X-ray apparatus such as an X-ray CT scanner and more particularly to an X-ray apparatus capable of emitting X-rays with high reliability, high efficiency and high-speed control.
For example, in a computerized tomograph apparatus which is widely used as a CT scanner, an industrial X-ray photograph apparatus for general medical treatment, or X-ray apparatus such as an X-ray exposure apparatus, a rotary anode type X-ray tube is used as an X-ray emission source in many cases. As is well known in the art, in the rotary anode type X-ray tube, a disk-like rotary anode is mechanically supported by a rotary structure and a stationary structure having a bearing disposed therebetween and a rotating driving power is supplied to a stator electromagnetic coil arranged outside a vacuum container corresponding to the position of the rotary structure so as to emit an electron beam from a cathode and apply the electron beam to the target surface of the rotary anode to emit X-ray while it is being rotated at high speed.
The bearing portion of the rotary anode type X-ray tube is constructed by an anti-friction bearing such as a ball bearing or a hydrodynamic pressure type slide bearing having a helical groove formed in the bearing surface and using a metal lubricant such as gallium (Ga) or gallium-indium-tin (Ga--In--Sn) alloy which is kept in the liquid form at least during the operation.
Examples of the rotary anode type X-ray tube using the latter hydrodynamic pressure type slide bearing are disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 60-21463 (U.S. Pat. No. 4,210,371), Jpn. Pat. Appln. KOKAI Publication No. 60-97536 (U.S. Pat. No. 4,562,587), Jpn. Pat. Appln. KOKAI Publication No. 60-117531 (U.S. Pat. No. 4,641,332), Jpn. Pat. Appln. KOKAI Publication No. 60-160552 (U.S. Pat. No. 4,644,577), Jpn. Pat. Appln. KOKAI Publication No. 62-287555 (U.S. Pat. No. 4,856,039), Jpn. Pat. Appln. KOKAI Publication No. 2-227947 (U.S. Pat. No. 5,068,885), or Jpn. Pat. Appln. KOKAI Publication No. 2-227948 (U.S. Pat. No. 5,077,775), for example.
The rotary anode type X-ray tube which is widely practiced in the prior art has a structure as shown in FIG. 1. That is, a disk-like rotary anode 11 is fixed on a shaft 12. The shaft 12 is fixed on a cylindrical rotary structure 13 which is formed of closely engaged iron and copper cylinders. The rotary structure 13 is fixed on a rotary shaft 14 arranged inside thereof. A cylindrical stationary structure 15 is arranged around the rotary shaft 14. A ball bearing 16 is arranged between the rotary shaft 14 and the stationary structure 15.
The disk-like rotary anode 11 has a thick base body 11a of molybdenum (Mo) and a thin target layer 11b formed of tungsten (W) alloy containing a small amount of rhenium (Re) on the inclined surface of the base body 11a.
When an X-ray photograph is taken by use of the X-ray apparatus using the rotary anode type X-ray tube with the above structure, an electron beam emitted from the cathode 17 is applied to the focal point track surface of the target layer 11b to emit X-ray (X) while the rotary anode 11 is being rotated at an anode rotation speed of 150 rps (revolutions per second) or more, for example. Heat generated in the portion of the target layer is transmitted to the Mo base body 11a and stored in the rotary anode, and at the same time, it is gradually radiated by radiation.
In recent years, in the CT scanner, for example, the operation for successively taking tomograms of a to-be-photographed object in a helical scanning mode for several tens of seconds, for example, is applied. When the X-ray is thus successively emitted from the rotary anode type X-ray tube for a long period of time, it often becomes necessary to limit the successive emission of the X-ray, particularly, because of a rise in the temperature of the anode of the X-ray tube. That is, the temperature of the rotary anode 11 of the X-ray tube varies such that the average temperature (Tf) of the focal point track area (F) indicated by broken lines at a certain time rises with the continuation time of the X-ray emission as schematically shown in FIGS. 2A and 2B. At the above certain time, the instantaneous temperature (Ts) of the electron beam incident point (S), that is, the X-ray focused point naturally reaches a temperature higher than the average temperature (Tf) of the focal point track area. Further, the average temperature (Tb) of the base body 11a is naturally set to a temperature lower than the average temperature (Tf) of the focal point track area. However, the temperatures of the respective portions rise with the continuation time of the X-ray emission.
The temperature (Tf) of the focal point track area indicates an average temperature of the focal point track area except the incident point (S) on which the electron beam is incident at a certain time, and the temperature (Ts) of the electron beam incident point indicates an achieved maximum temperature of the electron beam incident point at the instant. The average temperature (Tb) of the anode base body rises by heat storage or decreases by heat radiation according to a difference between the input heat quantity by the electron beam incident on the anode and the radiated heat quantity by heat radiation or the like.
The temperature (Ts) of the electron beam incident point becomes a peak temperature by an instantaneous input heat quantity by incidence of the electron beam in addition to the temperature (Tf) of the focal point track area only at the time of incidence of the electron beam. Further, the temperature (Ts) of the electron beam incident point is relatively and largely influenced by the anode rotation speed since the instantaneous heat storage action at the electron beam incident point becomes different depending on the rotation speed of the anode. That is, if the temperatures are compared with the focal point track area temperature (Tf) kept at the same value, the temperature (Ts) of the electron beam incident point reaches a higher temperature when the anode rotation speed is low and the temperature (Ts) of the electron beam incident point is set to a relatively low temperature when the anode rotation speed is high.
As is disclosed in TOSHIBA Review Vol. 37, No. 9, pp777 to 780, the temperatures of the respective portions of the rotary anode can be expressed by the following approximation. EQU Ts=Tf+(2.multidot.P.multidot.w.sup.-1/2)/[S.multidot.(.pi..multidot..rho..m ultidot.C.multidot..lambda..multidot.v).sup.-1/2 ]
where (P) indicates the power of the electron beam incident on the anode 11 or the anode input power, (w) indicates the electron beam width in the anode rotating direction (the radial direction of the anode) or the focal point size, (S) indicates the area of a surface on which the electron beam is incident, (.rho.) indicates the density of the material of the anode surface portion, (C) indicates the specific heat thereof, (.lambda.) indicates the thermal conductivity thereof, and (v) indicates the circumferential speed of the electron beam incident point.
Further, if a rapid temperature rise occurring at the focused position of the rotary anode target is set to (.DELTA.Ts) and a temperature rise occurring on average on the ring-like focal point track area is set to (.DELTA.Tf), then the following relation is obtained. EQU Ts=Tb+.DELTA.Tf+.DELTA.Ts=Tf+.DELTA.Ts .DELTA.Ts=(2.multidot.P.multidot.w.sup.-1/2)/[S.multidot.(.pi..multidot..r ho..multidot.C.multidot..lambda..multidot.v).sup.-1/2 ]
As is clearly understood from the above equations, the rapid temperature rise (.DELTA.Ts) occurring in the focused position of the rotary anode target is approximately proportional to the anode input power (P), approximately proportional to the square root of the focal point size, approximately inversely proportional to the electron beam incident area (S), and approximately inversely proportional to the square root of the rotation speed of the anode. On the other hand, it is known that heat radiation from the surface of, the rotary anode target is proportional to the absolute temperature of the anode target surface to the fourth power.
In the operation of the X-ray tube, the temperature rises in the respective portions of the rotary anode must be controlled so as not to cause evaporation, melting, deform of the anode material and damage of the connecting portion. If the target layer is formed of tungsten or tungsten alloy, for example, it is generally considered that the instantaneous temperature (Ts) of the focal point must be set to approx. 2800.degree. C. or less, (.DELTA.Tf) must be set in a range of approx. 100 to 500.degree. C., and (.DELTA.Ts) must be set in a range of approx. 1300 to 1500.degree. C. Therefore, the upper limit of the average temperature (Tb) of the anode base body is in fact considered to be approx. 1000.degree. C.
When the X-ray photographing is repeatedly effected under various X-ray emission conditions, it is practically difficult to actually and accurately measure the average temperature (Tb) of the anode base body, the focal point temperature (Ts) or the average temperature (Tf) of the focal point track area. This is because the measurement error in the average temperature (Tb) of the anode base body becomes large since a difference in the temperature distribution is large when the X-ray is emitted only for a short period of time. Further, the respective temperatures (Ts), (Tf) of the focal point areas are extremely high and significantly vary as described before, it is difficult to measure the temperatures with high precision and the measurement is strongly influenced by the X-ray emitting conditions such as the anode input power, focal point size, and anode rotation speed. Further, it is not impossible to calculate the respective temperatures by use of a computer, but it is impractical from the viewpoint of the calculation speed and cost of the computer.
Therefore, an X-ray apparatus constructed to control the X-ray emission based on the anode storage heat quantity (Hu) is widely used. As is well known in the art, the anode storage heat quantity (Hu) is expressed by the anode input power and the period of supply time thereof, that is, the product thereof with the continuation time of X-ray emission (Hu=kV.times.mA.times.T). Further, if the density of the material of the rotary anode target is set to (n), the specific heat is (C), the volume is (Vm) and the base body temperature is set to (Tb), then the heat quantity (Hu) of the anode target is approximated by Hu=.SIGMA.(.rho..times.C.times.Vm.times.Tb).
Therefore, since the base body temperature (Tb) is limited to approx. 1000.degree. C. as described before, the maximum permissible storage heat quantity of the anode target is determined as a value inherent to the rotary anode target. For this reason, it is a common practice to control and manage the anode storage heat quantity so as not to exceed a previously determined maximum permissible value. The rise and fall characteristics of the anode storage heat quantity of the mounted rotary anode type X-ray tube are shown in FIG. 3, for example, as is well known in the art. That is, the rise characteristic (St) of the anode storage heat quantity rises with the X-ray emission continuation time (T) and the rate of the rise becomes higher depending on the input power (P=anode peak voltage.times.anode average current) to the rotary anode. The maximum permissible storage heat quantity (Qlm) of the rotary anode is the upper limit heat quantity which can be safely stored in the anode and this value is set by taking the safety factor into consideration.
The cooling characteristic after the input to the anode, that is, the X-ray emission is terminated is a characteristic in which the anode storage heat quantity falls according to the cooling curve (Ct) inherent to the rotary anode type X-ray tube from the maximum permissible storage heat quantity (Qlm). That is, even if the achieved anode storage heat quantity is different, the heat quantity substantially falls according to the cooling curve (Ct).
As described before, since the characteristics of the anode storage heat quantity of the X-ray tube are inherent characteristics which the mounted X-ray tube has, they can be grasped substantially accurately according to the history of the ON and OFF states of the X-ray emission. Therefore, as shown in FIG. 4, the X-ray emission is controlled so that the anode storage heat quantity of the mounted X-ray tube will not exceed the maximum permissible storage heat quantity (Qlm). In FIG. 4, the period from the time t1 to t2 is the X-ray emission continuation time, the period from the time t2 to t3 is the cooling period, the period from the time t3 to t4 is the X-ray emission continuation period and the period after the time t4 is the cooling period.
Since it is possible to predict from the above characteristics that the X-ray photographing can be made under the predicted conditions such as the anode input power and the X-ray emission continuation time in the next cycle, a system for locking the apparatus so as not to permit the X-ray emission or similar control means is provided on the X-ray apparatus. The inventions related to the above technology are disclosed in the Patent Publication or Specification of Jpn. Pat. Appln. KOKAI Publication No. 57-5298, Jpn. Pat. Appln. KOKAI Publication No. 58-23199, Jpn. Pat. Appln. KOKAI Publication No. 59-217995, Jpn. Pat. Appln. KOKAI Publication No. 59-217996, Jpn. Pat. Appln. KOKAI Publication No. 62-69495, Jpn. Pat. Appln. KOKAI Publication No. 6-196113, U.S. Pat. No. 4,225,787, U.S. Pat. No. 4,426,720, and U.S. Pat. No. 5,140,246, for example.
As shown in FIG. 5A, the anode storage heat quantity is the same in a case (b) where the input power (P) to the anode is 20 kW and the X-ray emission continuation time is 50 sec and a case (c) where the anode input power (P) is 50 kW and the X-ray emission continuation time is 20 sec, for example, and the same value is used for control in the calculations for the conventional X-ray photographing control.
However, the temperature (Ts) of the electron beam incident point of the rotary anode and the average temperature (Tf) of the focal point track area reach temperatures higher than those attained based on the power ratio in a case where the anode input power (P) is larger as shown in FIG. 5C in comparison with a case where the anode input power (P) is smaller as shown in FIG. 5B. That is, the temperature (Tsc) of the electron beam incident point set 20 sec after the X-ray emission is started with the input power (P) of 50 kW reaches a temperature higher than 2.5 times which is the anode input power ratio in comparison with the temperature (Tsb) of the electron beam incident point set 50 sec after the X-ray emission is started with the input power (P) of 20 kW.
The reason is that a certain period of time is required for the heat conductivity or diffusion from the focused point of the rotary anode and the focal point track area to the anode base body and the temperature (Tf) of the focal point track area becomes excessively higher as the anode input power (P) is higher even if the anode input heat quantity (P.times.T) is the same, that is, it becomes rapidly higher than that determined by the ratio of the input power (P) in a short period of time. As a result, the temperature (Ts) of the electron beam incident point which is superposed thereon and attained becomes rapidly high in a short period of time. As described above, if the temperature (Ts) of the electron beam incident point becomes close to or exceeds the melting point of the focal point surface, the evaporation or melting phenomenon of the focal point surface material occurs to cause fatal damage.
Therefore, conventionally, in order to previously prevent the above problem, the maximum permissible storage heat quantity (Qlm) of the anode storage heat quantity shown in FIG. 4 is determined to a relatively low value by taking the above phenomenon in a case where the anode input power (P) is highest into consideration and taking the sufficiently large safety factor. According to this, the X-ray apparatus can be safely operated without causing any damage on the rotary anode even if the assumable highest anode input power is used. However, in the case of low anode input power, the control operation is performed so as not to permit the next X-ray emission until the anode is cooled to a temperature than necessary. Thus, in the conventional X-ray apparatus, the wait time for the next X-ray emission becomes unnecessarily longer in many cases and the performance of the mounted X-ray tube cannot be fully utilized.
In a conventional X-ray apparatus including an X-ray tube having a rotary anode with a laminated structure of a graphite base body soldered, for example, on the rear surface of the relatively thin Mo base body, the heat conductivity from the focal point track area to the graphite base body is worsen, the melting point of solder is low, and the soldered portion tends to be separated and the maximum permissible storage heat quantity (Qlm) of the anode storage heat quantity is set to a smaller value.
An object of this invention is to provide an X-ray apparatus which can be automatically controlled with high speed and high reliability and always utilize the performance of a mounted X-ray tube, that is, the heat quantity to the maximum extent, and always suppress the wait time for the next X-ray photographing, that is, X-ray emission to minimum.